Multi-plasma nitridation process for a gate dielectric

ABSTRACT

A gate dielectric can be formed by depositing a first silicon oxide material by a first atomic layer deposition process. The thickness of the first silicon oxide material is selected to correspond to at least 10 deposition cycles of the first atomic layer deposition process. The first silicon oxide material is converted into a first silicon oxynitride material by a first plasma nitridation process. A second silicon oxide material is subsequently deposited by a second atomic layer deposition process. The second silicon oxide material is converted into a second silicon oxynitride material by a second plasma nitridation process. Multiple repetitions of the atomic layer deposition process and the plasma nitridation process provides a silicon oxynitride material having a ratio of nitrogen atoms to oxygen atoms greater than 1/3, which can be advantageously employed to reduce the leakage current through a gate dielectric.

BACKGROUND

The present disclosure relates to a semiconductor manufacturing process,and particularly to a method of forming a gate dielectric through amulti-plasma nitridation process in combination with silicon oxidedeposition by atomic layer deposition.

Gate oxide reliability is one of major reliability issues that advancedsemiconductor manufacturing technology faces. The gate oxide of a fieldeffect transistor needs to be as thin as possible while avoidingdielectric breakdown caused by leakage currents. An equivalent gateoxide thickness calculated by capacitance measurements tracks with anequivalent gate oxide thickness calculated by measurements on leakagecurrents, with the offset being substantially independent of mostprocessing parameters employed to form the gate dielectric. A method ofreducing the equivalent gate oxide thickness calculated by measurementson leakage currents without reducing the equivalent gate oxide thicknesscalculated by capacitance measurements is desired.

BRIEF SUMMARY

A gate dielectric can be formed by depositing a first silicon oxidematerial by a first atomic layer deposition process. The thickness ofthe first silicon oxide material is selected to correspond to at least10 deposition cycles of the first atomic layer deposition process. Thefirst silicon oxide material is converted into a first siliconoxynitride material by a first plasma nitridation process. A secondsilicon oxide material is subsequently deposited by a second atomiclayer deposition process. The second silicon oxide material is convertedinto a second silicon oxynitride material by a second plasma nitridationprocess. Multiple repetitions of the atomic layer deposition process andthe plasma nitridation process provides a silicon oxynitride materialhaving a ratio of nitrogen atoms to oxygen atoms greater than 1/3, whichcan be advantageously employed to reduce the leakage current through agate dielectric.

According to an aspect of the present disclosure, a method of forming asemiconductor structure is provided. A first silicon oxide material isdeposited on a semiconductor substrate by a first atomic layerdeposition process. The first silicon oxide material is converted into afirst silicon oxynitride material by a first plasma nitridation process.A second silicon oxide material is deposited on the first siliconoxynitride material by a second atomic layer deposition process. Thesecond silicon oxide material is converted into a second siliconoxynitride material by a second plasma nitridation process. A ratio ofnitrogen atoms to oxygen atoms in a stack of the first and secondsilicon oxynitride materials is greater than 1/3.

According to another aspect of the present disclosure, a method offorming a semiconductor structure is provided. A first gate dielectricand a second gate dielectric are formed over a first channel region andover a second channel region, respectively, in a semiconductorsubstrate. The first channel region and the second channel region havedifferent compositions, and the first and second gate dielectrics have asubstantially same thickness over the first channel region and thesecond channel region.

According to yet another aspect of the present disclosure, asemiconductor structure includes a first channel region location in asemiconductor substrate, a second channel region located in thesemiconductor substrate, a first gate dielectric located over the firstchannel region, and a second gate dielectric located over the secondchannel region. The first and second channel regions have differentcompositions. The first and second gate dielectrics have substantiallysame thickness. In one embodiment, the first channel region includessilicon, and the second channel region includes a silicon-germaniumalloy including germanium at an atomic concentration greater than about20%, and a difference between thicknesses of the second gate dielectricand the first gate dielectric is less than, or equal to, about 0.3 nm.

According to still another aspect of the present disclosure, asemiconductor structure is provided, which includes a first gatedielectric and a second gate dielectric. The first gate dielectric islocated on a silicon portion in a semiconductor substrate and includes afirst vertical stack, from bottom to top, of a silicon oxynitride layerincluding nitrogen at an atomic concentration in a range from 2% to 10%and a first silicon oxynitride material portion having anitrogen-to-oxygen atomic ratio greater than 1/3. The second gatedielectric is located on a silicon-germanium alloy portion in thesemiconductor substrate and includes a second vertical stack, frombottom to top, of a silicon-germanium oxynitride layer includingnitrogen at an atomic concentration in a range from 2% to 10% and asecond silicon oxynitride material portion having a samenitrogen-to-oxygen atomic ratio as the first silicon oxynitride materialportion. In one embodiment, the silicon-germanium alloy portion includesgermanium at an atomic concentration greater than 20%, and a differencebetween a thickness of the second vertical stack and the first verticalstack is not greater than 0.3 nm.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of an exemplary semiconductorstructure after formation of shallow trench isolation structuresaccording to an embodiment of the present disclosure.

FIG. 2 is a vertical cross-sectional view of the exemplary semiconductorstructure after formation of silicon-containing oxynitride materialportions according to an embodiment of the present disclosure.

FIG. 3 is vertical cross-sectional view of the exemplary semiconductorstructure after deposition of a first silicon oxide material accordingto an embodiment of the present disclosure.

FIG. 4 is a vertical cross-sectional view of the exemplary semiconductorstructure after conversion of the first silicon oxide material into afirst silicon oxynitride material according to an embodiment of thepresent disclosure.

FIG. 5 is a vertical cross-sectional view of the exemplary semiconductorstructure after deposition of a second silicon oxide material accordingto an embodiment of the present disclosure.

FIG. 6 is a vertical cross-sectional view of the exemplary semiconductorstructure after conversion of the second silicon oxide material into asecond silicon oxynitride material according to an embodiment of thepresent disclosure.

FIG. 7 is a vertical cross-sectional view of the exemplary semiconductorstructure after formation of field effect transistors according to anembodiment of the present disclosure.

FIG. 8 is a graph illustrating the relationship between a leakagecurrent equivalent dielectric thickness and an inversion based gatedielectric thickness of gate dielectrics including various ratios fornitrogen-to-oxygen atomic ratios according to an embodiment of thepresent disclosure.

FIG. 9 is a vertical cross-sectional view of a variation of theexemplary semiconductor structure after formation of an additionalsilicon oxynitride material according to an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

As stated above, the present disclosure relates to a method of forming agate dielectric through a multi-plasma nitridation process incombination with silicon oxide deposition by atomic layer deposition.Aspects of the present disclosure are now described in detail withaccompanying figures Like and corresponding elements mentioned hereinand illustrated in the drawings are referred to by like referencenumerals. The drawings are not necessarily drawn to scale.

Referring to FIG. 1, an exemplary semiconductor structure according toan embodiment of the present disclosure includes a substrate 8, which isa semiconductor substrate, i.e., a substrate that includes asemiconductor material. The substrate 8 can be asemiconductor-on-insulator (SOI) substrate, a bulk semiconductorsubstrate, or a hybrid substrate including a bulk portion and an SOIportion. In one embodiment, the substrate 8 can be an SOI substrateincluding a handle substrate 10, a buried insulator layer 20, and a topsemiconductor layer.

Shallow trench isolation structures 22 can be formed in the topsemiconductor layer employing methods known in the art. In this case,the remaining portions of the top semiconductor layer can be dividedinto multiple top semiconductor portions that are laterally spaced bythe shallow trench isolation structures. For example, the multiple topsemiconductor portions can include a first top semiconductor portion 30Aand a second top semiconductor portion 30B.

The first top semiconductor portion 30A and the second top semiconductorportion 30B can include the same semiconductor material, or can includedifferent semiconductor materials. The semiconductor materials that canbe employed for the multiple top semiconductor portions (30A, 30B)include, but are not limited to, silicon, a silicon-germanium alloy, asilicon-carbon alloy, a silicon-germanium-carbon alloy, compoundsemiconductor materials, and combinations and/or layered stacks of theforegoing. In an illustrative example, the first top semiconductorportion 30A can include single crystalline silicon, and the second topsemiconductor portion 30B can include a single crystalline materialincluding a silicon-germanium alloy, a silicon-carbon alloy, or asilicon-germanium-carbon alloy. The top semiconductor portions (30A,30B) can be single crystalline, polycrystalline, or amorphous. In oneembodiment, the top semiconductor portions (30A, 30B) can be singlecrystalline.

While description of the present disclosure employs an embodiment inwhich the substrate 8 is an SOI substrate, embodiments in which thesubstrate is a bulk substrate or a hybrid substrate are expresslycontemplated herein.

Referring to FIG. 2, silicon-containing oxynitride material portions 52′can be formed by conversion of surface regions of the top semiconductorportions (30A, 30B) if the top semiconductor portions (30A, 30B) includeat least one silicon-containing material. The silicon containingmaterial can be, for example, silicon, a silicon-germanium alloy, asilicon-carbon alloy, or a silicon-germanium-carbon alloy. The topsemiconductor portions (30A, 30B) can be silicon-containing materialportions, and the conversion of the surface regions of the topsemiconductor portions (30A, 30B) can be performed by an anneal in anenvironment including an oxygen-and-nitrogen-containing gas and/or acombination of a nitrogen-containing gas and an oxygen-containing gas atan elevated temperature in a range from 700 degrees Celsius to 1,100degrees Celsius. The oxygen-and-nitrogen-containing gas is an inorganicgas that includes at least one oxygen atom and at least one nitrogenatom per molecule, and can be, for example, N₂O, NO, or NO₂. Theoxygen-containing gas and the nitrogen-containing gas may be the same(i.e., can be a gas including oxygen and nitrogen such as N₂O, NO, orNO₂), or can be two distinct gas species. In one embodiment, thenitrogen-containing gas can be N₂, N₂O, NO, or NO₂ and theoxygen-containing gas can be O₂, H₂O, NO, or NO₂. An inert carrier gassuch as Ar may be optionally employed during the anneal.

The anneal oxynitridates, i.e., simultaneously oxidizes and nitridates,the surface regions of the top semiconductor portions (30A, 30B), andforms silicon-containing oxynitride material portions 52′. Eachsilicon-containing oxynitride material portion 52′ can be, for example,a thermal silicon oxynitride portion or a thermalsilicon-germanium-oxynitride portion. The thickness of eachsilicon-containing oxynitride material portion 52′ can be from 0.7 nm to1.2 nm, although lesser and greater thicknesses can also be employed. Inone embodiment, at least one of the silicon-containing oxynitridematerial portions 52′ can consist essentially of thermal siliconoxynitride, and at least another of the silicon-containing oxynitridematerial portions 52′ can consist essentially of thermalsilicon-germanium oxynitride.

The oxynitridation of the surface regions of the top semiconductorportions (30A, 30B) incorporates nitrogen atoms into thesilicon-containing oxynitride material portions 52′ at an atomicconcentration in a range from 2% to 10%. The presence of nitrogen atomsin the silicon-containing oxynitride material portions 52′ has anadvantageous effect of reducing the difference among the conversionrates of silicon-germanium alloy materials having different atomicconcentrations of germanium in a range greater than 0% and less than100%, and between silicon and any silicon-germanium alloy materialhaving an atomic concentration of germanium in a range greater than 0%and less than 100%.

In one embodiment, the first top semiconductor portion 30A can be asilicon portion and the second top semiconductor portion 30B can be asilicon-germanium alloy portion. A surface region of the silicon portioncan be converted into a silicon oxynitride layer, and a surface regionof the silicon-germanium alloy portion can be converted into asilicon-germanium oxynitride layer simultaneously with the conversion ofthe surface region of the silicon portion. The simultaneously conversionof the surface region of the silicon portion and the surface region ofthe silicon-germanium alloy portion can be performed by a thermaloxynitridation process employing an oxygen-and-nitrogen-containing gasor a combination of an oxygen-containing gas and a nitrogen-containinggas. In one embodiment, the silicon-germanium alloy portion can includegermanium at an atomic concentration greater than 20%. In oneembodiment, the silicon-germanium alloy portion can include germanium atan atomic concentration in a range from 20% to 30%.

For example, if a thermal oxidation process that forms more than 0.5 nmof silicon oxide on a single crystalline silicon surface is employed toform a silicon-germanium oxide from a single crystallinesilicon-germanium alloy having an atomic concentration of germaniumgreater than 20%, the thickness of the silicon-germanium oxide isgreater than the thickness of the silicon oxide by at least 0.5 nm. Thethickness differential increases with the atomic concentration ofgermanium. In contrast, the oxynitridation process of the presentdisclosure can be employed to simultaneously form a silicon oxynitridehaving a first thickness on a single crystalline silicon material and asilicon-germanium oxynitride having a second thickness on a singlecrystalline silicon-germanium alloy material having an atomicconcentration not less than 20% and not greater than 30% such that thedifference between the second thickness and the first thickness does notexceed 0.3 nm.

Thus, if the first top semiconductor portion 30A includes singlecrystalline silicon, and if the second top semiconductor portion 30Bincludes a single crystalline silicon-germanium alloy in which theatomic concentration of germanium in a range from 20% and 30%, thethickness differential between the silicon-germanium oxynitride in thesilicon-containing oxynitride material portion 52′ on the second topsemiconductor portion 30B and the silicon oxynitride in thesilicon-containing oxynitride material portion 52′ in the first topsemiconductor portion 30A can be in a range from 0 nm to 0.3 nm when thethickness of the silicon oxynitride in the silicon-containing oxynitridematerial portion 52′ is in a range from 0.7 nm to 1.2 nm.

Referring to FIG. 3, a first silicon oxide material layer 54′ is formedon the top surface of the silicon-containing oxynitride materialportions 52′ by deposition of a first silicon oxide material. Thedeposition of the first silicon oxide material can be effected, forexample, by a first atomic layer deposition (ALD) process. During thefirst ALD process, the exemplary semiconductor structure is placedwithin a process chamber compatible with maintenance of an ultrahighvacuum environment. As used herein, an ultrahigh vacuum environmentrefers to an environment having a base pressure of less than 1.0×10⁻⁶Torr. In one embodiment, the base pressure of the process chamber can beless than 1.0×10⁻⁷ Torr. In another embodiment, the base pressure of theprocess chamber can be less than 1.0×10⁻⁸ Torr. In yet anotherembodiment, the base pressure of the process chamber can be less than1.0×10⁻⁹ Torr.

Upon loading of the exemplary semiconductor structure in the processchamber, the process chamber is pumped down to the base pressure.Subsequently, at least 10 deposition cycles are performed to deposit thefirst silicon oxide material on the surfaces of the silicon-containingoxynitride material portions 52′ and on physically exposed surfaces ofthe shallow trench isolation structures 22. Each deposition cycleincludes a silicon-containing precursor exposure phase in which theprocess chamber containing the exemplary semiconductor substrateincludes a silicon-containing precursor gas at a first pressure, and anoxygen exposure phase in which the process chamber includes anoxygen-containing gas at a second pressure.

Each pair of a silicon-containing precursor exposure phase and an oxygenexposure phase is temporally spaced by an intra-cycle pump down phase,during which the process chamber is pumped down to the base pressure.Each deposition cycle is temporally spaced from a preceding depositioncycle or from a subsequent deposition cycle by an inter-cycle pump downphase, during which the process chamber is pumped down to the basepressure. Thus, the process chamber is void of, i.e., does not include,any oxygen-containing gas during the silicon-containing precursor phase,and is void of any silicon-containing gas during the oxygen exposurephase.

The silicon-containing precursor gas is a gas that includes at least onesilicon atom per molecule. In one embodiment, each molecule of thesilicon-containing precursor gas can include at least one silicon atomand at least one hydrogen atom. In one embodiment, thesilicon-containing precursor gas can be selected from, for example,SiH₄, SiH₂Cl₂, SiHCl₃, and Si₂H₆. The oxygen-containing gas is a gasthat includes at least one oxygen atom per molecule, and can be selectedfrom, for example, O₂, O₃, and a combination of O₂ and O₃.

The duration of each silicon-containing precursor exposure phase can befrom 1 second to 60 seconds, although lesser and greater durations canalso be employed. The duration of each oxygen exposure phase can be from1 second to 60 seconds, although lesser and greater durations can alsobe employed.

The partial pressure of the silicon-containing precursor gas during eachsilicon-containing precursor exposure phase can be in a range from1.0×10⁻⁵ Torr to 1.0×10⁻² Torr, although lesser and greater partialpressures can also be employed. The partial pressure of theoxygen-containing gas during each oxygen exposure phase can be in arange from 1.0×10⁻⁵ Torr to 1.0×10⁻² Torr, although lesser and greaterpartial pressures can also be employed.

The temperature of the exemplary semiconductor structure during thefirst ALD process can be in a range from 200 degrees Celsius to 600degrees Celsius, although lesser and greater temperatures can also beemployed. In one embodiment, the temperature of the exemplarysemiconductor structure during the first ALD process can be in a rangefrom 300 degrees Celsius to 450 degrees Celsius.

Each deposition cycle deposits a monolayer of silicon oxide material,which has a thickness of about 0.15 nm. The thickness of the firstsilicon oxide material layer 54′ is linearly proportional to the numberof deposition cycles in the first ALD process. In one embodiment, thefirst atomic layer deposition process employs a number of depositioncycles selected from a range from 12 to 20, which corresponds to athickness range from 1.8 nm to 3.0 nm. In another embodiment, the firstatomic layer deposition process employs a number of deposition cyclesselected from a range from 13 to 17, which corresponds to a thicknessrange from 1.95 nm to 2.55 nm.

The thickness of the first silicon oxide material layer 54′ is selectedto be greater than a minimum thickness in order to avoid formation ofsilicon nitride at the interface between the top semiconductor portions(30A, 30B) and the silicon-containing oxynitride material portions 52′during a plasma nitridation process to be subsequently performed. Theminimum thickness of the first silicon oxide material layer 54′ dependson the thickness of the silicon-containing oxynitride material portions52′ and the intensity and duration of the nitrogen plasma to be employedin the subsequent plasma nitridation process. It is known that formationof a semiconductor nitride on a surface of a semiconductor portioninduces pinning of charge carriers, and degrades the performance of thesemiconductor portion as a channel of a field effect transistor.

In general, a total thickness of about 2.7 nm is needed for thecombination of the silicon-containing oxynitride material portions 52′and the first silicon oxide material layer 54′ in order to avoidformation of silicon nitride or a silicon-germanium nitride at theinterface between the top semiconductor portions (30A, 30B) and thesilicon-containing oxynitride material portions 52′ during thesubsequent plasma nitridation process. Because a thickness of about 0.9nm is necessary to ensure formation of a contiguous silicon oxide layerduring formation of the silicon-containing oxynitride material portions52′, the first silicon oxide material layer 54′ needs to have athickness of at least 1.8 nm, which corresponds to 12 deposition cycles.

Referring to FIG. 4, the exemplary semiconductor structure is removedfrom the process chamber for the first ALD process, and is placed in aprocess chamber configured for a plasma nitridation process, which isherein referred to as a plasma nitridation chamber. A first plasmanitridation process is performed to introduce nitrogen atoms in thestack of the first silicon oxide material layer 54′ and thesilicon-containing oxynitride material portions 52′. The first plasmanitridation process converts the first silicon oxide material of thefirst silicon oxide material layer 54′ into a first silicon oxynitridematerial layer 54 containing a silicon oxynitride material, and convertsthe silicon-containing oxide material(s) of the silicon-containingoxynitride material portions 52′ into silicon-containing oxynitridematerial portions 52 containing silicon-containing oxynitridematerial(s).

The first plasma nitridation process employs a plasma of anitrogen-containing gas. The nitrogen-containing gas is a gas includingat least one nitrogen atom per molecule. In one embodiment, thenitrogen-containing gas can be selected from N₂, NH₃, NO, N₂O, NO₂, andcombinations thereof. In one embodiment, the first plasma nitridationprocess can employ a plasma generation chamber coupled to the processchamber in which the semiconductor substrate is placed through a plasmaconduit. The configuration in which the plasma generation chamber andthe process chamber are connected through the plasma conduit is referredto as a “decoupled plasma” configuration. The nitrogen plasma isgenerated in the plasma generation chamber, flows through the plasmaconduit into the process chamber, and nitridates the first silicon oxidematerial layer 54′ and the silicon-containing oxynitride materialportions 52′ to form the first silicon oxynitride material layer 54 andthe silicon-containing oxynitride material portions 52, respectively.

The first silicon oxide material layer 54′ is directly exposed to theplasma of the nitrogen-containing gas, and the silicon-containingoxynitride material portions 52′ are not directly exposed to the plasmaof the nitrogen-containing gas. Nitrogen atoms that are incorporatedinto the silicon-containing oxynitride material portions 52 pass throughthe first silicon oxide material layer 54′. Thus, the averageconcentration of nitrogen in the silicon-containing oxynitride materialportions 52 is lesser than the average concentration of the firstsilicon oxynitride material layer 54. The thickness of thesilicon-containing oxynitride material portions 52′ and the thickness ofthe first silicon oxide material layer 54′ can be selected to avoidformation of a semiconductor nitride at the interface between the topsemiconductor portions (30A, 30B) and the silicon-containing oxynitridematerial portions 52 as discussed above.

In a non-limiting illustrative example, the substrate 8 can be a 300 mmsemiconductor substrate as known in the art. In this case, the firstplasma nitridation process can be performed employing a radio frequency(RF) power for plasma generation in a range from 1 kW to 3 kW, a dutycycle in a range from 20% to 50%, plasma pressure in a range from 1mTorr to 30 mTorr, temperature in a range from 10 degrees Celsius to 600degrees Celsius, and a plasma duration in a range from 1 minute to 20minutes.

Optionally, the semiconductor substrate with the silicon-containingoxynitride material portions 52 and the first silicon oxynitridematerial layer 54 thereupon can be placed in a nitridation environmentafter the first plasma nitridation process. The nitridation environmentcan include a nitrogen-containing gas at an elevated temperature in arange from 600 degrees Celsius to 900 degrees Celsius. The nitridationenvironment can be provided, for example, by a conventional furnace thatoperates at atmospheric temperature. The duration of the nitridation atthe elevated temperature can be from 1 minute to 60 minutes.

In one embodiment, the ratio of nitrogen atoms to oxygen atoms in thestack of the silicon-containing oxynitride material portions 52 and thefirst silicon oxynitride material layer 54 can be greater than 1/3 afterthe processing step(s) of FIG. 4. In one embodiment, the ratio ofnitrogen atoms to oxygen atoms in the stack of the silicon-containingoxynitride material portions 52 and the first silicon oxynitridematerial layer 54 can be in a range from 1/3 to 0.70. In anotherembodiment, the ratio of nitrogen atoms to oxygen atoms can be in arange from 0.45 to 0.70. In yet another embodiment, the ratio ofnitrogen atoms to oxygen atoms can be in a range from 0.50 to 0.65.

In one embodiment, the stack of the silicon-containing oxynitridematerial portions 52 and the first silicon oxynitride material layer 54can have an overall composition of SiO_(x)N_(y), in which x/2+3y/4 is ina range from 0.75 to 1.25. In another embodiment, the stack of thesilicon-containing oxynitride material portions 52 and the first siliconoxynitride material layer 54 can have an overall composition ofSiO_(x)N_(y), in which x/2+3y/4 is in a range from 0.95 to 1.05 . In yetanother embodiment, the stack of the silicon-containing oxynitridematerial portions 52 and the first silicon oxynitride material layer 54can have an overall composition of SiO_(x)N_(y), in which x/2+3y/4 issubstantially 1.00, i.e., in a range from 0.99 to 1.01.

The atomic percentage of nitrogen in the silicon-containing oxynitridematerial portions 52 is lower than the average atomic percentage ofnitrogen in the stack of the silicon-containing oxynitride materialportions 52 and the first silicon oxynitride material layer 54. In oneembodiment, the ratio of nitrogen atoms to oxygen atoms in thesilicon-containing oxynitride material portions 52 is less than theaverage ratio of nitrogen atoms to oxygen atoms in the stack of thesilicon-containing oxynitride material portions 52 and the first siliconoxynitride material layer 54. The ratio of nitrogen atoms to oxygenatoms in the silicon-containing oxynitride material portions 52 can beless than, equal to, or greater than, 1/3. In one embodiment, the ratioof nitrogen atoms to oxygen atoms in the silicon-containing oxynitridematerial portions 52 can be in a range from 0.1 to 0.50. In anotherembodiment, the ratio of nitrogen atoms to oxygen atoms can be in arange from 0.15 to 1/3. In yet another embodiment, the ratio of nitrogenatoms to oxygen atoms can be in a range from 1/3 to 0.50.

In one embodiment, the silicon-containing oxynitride material portions52 can have an overall composition of SiO_(x)N_(y), in which x/2+3y/4 isin a range from 0.75 to 1.25. In another embodiment, thesilicon-containing oxynitride material portions 52 can have an overallcomposition of SiO_(x)N_(y), in which x/2+3y/4 is in a range from 0.95to 1.05. In yet another embodiment, the silicon-containing oxynitridematerial portions 52 can have an overall composition of SiO_(x)N_(y), inwhich x/2+3y/4 is substantially 1.00, i.e., in a range from 0.99 to1.01.

Referring to FIG. 5, a second silicon oxide material layer 56′ is formedon the top surface of the first silicon oxynitride material layers 54 bydeposition of a second silicon oxide material. The deposition of thesecond silicon oxide material can be effected, for example, by a secondatomic layer deposition (ALD) process. During the second ALD process,the exemplary semiconductor structure is placed within a process chambercompatible with maintenance of an ultrahigh vacuum environment. In oneembodiment, the same process chamber can be employed for the second ALDprocess as for the first ALD process.

Upon loading of the exemplary semiconductor structure in the processchamber, the process chamber is pumped down to the base pressure.Subsequently, a plurality of deposition cycles is performed to depositthe second silicon oxide material on the surfaces of the first siliconoxynitride material layers 54. Each deposition cycle includes asilicon-containing precursor exposure phase in which the process chambercontaining the exemplary semiconductor substrate includes asilicon-containing precursor gas at a second pressure, and an oxygenexposure phase in which the process chamber includes anoxygen-containing gas at a second pressure.

Each pair of a silicon-containing precursor exposure phase and an oxygenexposure phase is temporally spaced by an intra-cycle pump down phase,during which the process chamber is pumped down to the base pressure.Each deposition cycle is temporally spaced from a preceding depositioncycle or from a subsequent deposition cycle by an inter-cycle pump downphase, during which the process chamber is pumped down to the basepressure. Thus, the process chamber is void of any oxygen-containing gasduring the silicon-containing precursor phase, and is void of anysilicon-containing gas during the oxygen exposure phase.

The silicon-containing precursor gas can be the same as in the first ALDprocess. The oxygen-containing gas can be the same as in the first ALDprocess. Processing parameters of the silicon-containing precursorexposure phase and the oxygen exposure phase can be within the samerange as in the first ALD process.

Each deposition cycle deposits a monolayer of silicon oxide material.The thickness of the second silicon oxide material layer 56′ is linearlyproportional to the number of deposition cycles in the second ALDprocess. In one embodiment, the second ALD process can include at least4 deposition cycles. In one embodiment, the second ALD process canemploy a number of deposition cycles selected from a range from 4 to 20,which corresponds to a thickness range from 0.6 nm to 3.0 nm. In anotherembodiment, the second atomic layer deposition process employs a numberof deposition cycles selected from a range from 4 to 6, whichcorresponds to a thickness range from 0.6 nm to 0.9 nm.

Referring to FIG. 6, the exemplary semiconductor structure is removedfrom the process chamber for the second ALD process, and is placed in aplasma nitridation chamber. A second plasma nitridation process isperformed to introduce nitrogen atoms into the second silicon oxidematerial layer 56′. The second plasma nitridation process converts thesecond silicon oxide material of the second silicon oxide material layer56′ into a second silicon oxynitride material layer 56 containing asilicon oxynitride material.

The second plasma nitridation process employs a plasma of anitrogen-containing gas. Any of the nitrogen-containing gas that can beemployed for the first plasma nitridation process can be employed forthe second plasma nitridation process. In one embodiment, the secondplasma nitridation process can employ a combination of a plasmageneration chamber and a process chamber in a decoupled plasmaconfiguration. The nitrogen plasma is generated in the plasma generationchamber, flows through the plasma conduit into the process chamber, andnitridates the second silicon oxide material layer 56′ to form thesecond silicon oxynitride material layer 56.

The second silicon oxide material layer 56′ is directly exposed to theplasma of the nitrogen-containing gas. The intensity and the duration ofthe nitridation plasma can be selected to achieve a target concentrationof nitrogen in the second silicon oxynitride material layer 56, anddepends on the thickness of the second silicon oxide material layer 56′to be nitridated.

In a non-limiting illustrative example, the substrate 8 can be a 300 mmsemiconductor substrate as known in the art. In this case, the secondplasma nitridation process can be performed employing a radio frequency(RF) power for plasma generation in a range from 0.7 kW to 2 kW, a dutycycle in a range from 20% to 50%, plasma pressure in a range from 1mTorr to 30 mTorr, temperature in a range from 10 degrees Celsius to 600degrees Celsius, and a plasma duration in a range from 20 seconds to 10minutes.

In one embodiment, the ratio of nitrogen atoms to oxygen atoms in thestack of the silicon-containing oxynitride material portions 52, thefirst silicon oxynitride material layer 54, and the second siliconoxynitride material layer 56 can be greater than 1/3. In one embodiment,the ratio of nitrogen atoms to oxygen atoms in the stack of thesilicon-containing oxynitride material portions 52, the first siliconoxynitride material layer 54, and the second silicon oxynitride materiallayer 56 can be in a range from 1/3 to 0.70. In another embodiment, theratio of nitrogen atoms to oxygen atoms can be in a range from 0.45 to0.70. In yet another embodiment, the ratio of nitrogen atoms to oxygenatoms can be in a range from 0.50 to 0.65.

In one embodiment, the stack of the silicon-containing oxynitridematerial portions 52, the first silicon oxynitride material layer 54,and the second silicon oxynitride material layer 56 can have an overallcomposition of SiO_(x)N_(y), in which x/2+3y/4 is in a range from 0.75to 1.25. In another embodiment, the stack of the silicon-containingoxynitride material portions 52, the first silicon oxynitride materiallayer 54, and the second silicon oxynitride material layer 56 can havean overall composition of SiO_(x)N_(y), in which x/2+3y/4 is in a rangefrom 0.95 to 1.05. In yet another embodiment, the stack of thesilicon-containing oxynitride material portions 52, the first siliconoxynitride material layer 54, and the second silicon oxynitride materiallayer 56 can have an overall composition of SiO_(x)N_(y), in whichx/2+3y/4 is substantially 1.00, i.e., in a range from 0.99 to 1.01.

The atomic percentage of nitrogen in the silicon-containing oxynitridematerial portions 52 is lower than the average atomic percentage ofnitrogen in the stack of the silicon-containing oxynitride materialportions 52, the first silicon oxynitride material layer 54, and thesecond silicon oxynitride material layer 56. In one embodiment, theratio of nitrogen atoms to oxygen atoms in the silicon-containingoxynitride material portions 52 is less than average ratio of nitrogenatoms to oxygen atoms in the stack of the silicon-containing oxynitridematerial portions 52, the first silicon oxynitride material layer 54,and the second silicon oxynitride material layer 56. The ratio ofnitrogen atoms to oxygen atoms in the silicon-containing oxynitridematerial portions 52 can be less than, equal to, or greater than, 1/3.In one embodiment, the ratio of nitrogen atoms to oxygen atoms in thesilicon-containing oxynitride material portions 52 can be in a rangefrom 0.1 to 0.50. In another embodiment, the ratio of nitrogen atoms tooxygen atoms can be in a range from 0.15 to 1/3. In yet anotherembodiment, the ratio of nitrogen atoms to oxygen atoms can be in arange from 1/3 to 0.50.

In one embodiment, the silicon-containing oxynitride material portions52 can have an overall composition of SiO_(x)N_(y), in which x/2+3y/4 isin a range from 0.75 to 1.25. In another embodiment, thesilicon-containing oxynitride material portions 52 can have an overallcomposition of SiO_(x)N_(y), in which x/2+3y/4 is in a range from 0.95to 1.05. In yet another embodiment, the silicon-containing oxynitridematerial portions 52 can have an overall composition of SiO_(x)N_(y), inwhich x/2+3y/4 is substantially 1.00, i.e., in a range from 0.99 to1.01.

The silicon-containing oxynitride material portions 52, the firstsilicon oxynitride material layer 54, and the second silicon oxynitridematerial layer 56 collectively constitute a gate dielectric layer 50L.

Referring to FIG. 7, field effect transistors can be formed on thesubstrate 8 by forming gate stacks (50, 58), gate spacers 59, and sourceand drain regions. For example, a gate conductor layer can be depositedover the gate dielectric layer 50. The gate conductor layer includes atleast one conductive material, which can be at least one metallicmaterial, at least one doped semiconductor material, or a combinationthereof. The gate conductor layer and the gate dielectric layer 50L canbe patterned by a combination of lithographic patterning of an appliedphotoresist layer and transfer of the pattern in the photoresist layerinto the gate conductor layer and the gate dielectric layer 50L by atleast one anisotropic etch that employs the photoresist layer as an etchmask. The remaining portions of the gate dielectric layer 50L constitutegate dielectrics 50, and remaining portions of the gate conductor layerconstitutes gate electrodes 58. The gate spacers 59 can be formed, forexample, by deposition of a conformal dielectric material layer and by asubsequent anisotropic etch that removes horizontal portions of theconformal dielectric material layer. The remaining vertical portions ofthe conformal dielectric material layer constitute the gate spacers 58.The source and drain regions can be formed, for example, by implantingelectrical dopants (which can be p-type dopants or n-type dopants) intoregions of the top semiconductor portions (30A, 30B) employing the gatestacks (50, 58) and the gate spacers 59 as an implantation mask. Theimplanted regions of the first top semiconductor portion 30A can becomea first source region 34A and a first drain region 36A, and theunimplanted region of the first top semiconductor portion 30A can becomea first body region 32A. The implanted regions of the second topsemiconductor portion 30B can become a second source region 34B and asecond drain region 36B, and the unimplanted region of the second topsemiconductor portion 30B can become a second body region 32B. Othermethods for forming field effect transistors as known in the art mayalso be employed.

In one embodiment, the first body region 32A can be a silicon portionand the second body region 32B can be a silicon-germanium alloy portion.The silicon-containing oxynitride material portions 52 on the first bodyregion 32A can be a silicon oxynitride layer, and the silicon-containingoxynitride material portions 52 on the second body region 32B can be asilicon-germanium oxynitride layer. In one embodiment, the first bodyregion 32A can consist essentially of silicon and optional electricaldopants (p-type dopants or n-type dopants), and the second body regioncan consist essentially of a silicon-germanium alloy including germaniumat an atomic concentration in a range from 0.1% to 99.9%. A first stackof the silicon oxynitride layer, a first portion of the first siliconoxynitride material, and a first portion of the second siliconoxynitride material can be patterned into a first gate dielectric, i.e.,the gate dielectric layer 50 on the first body region 32A. A secondstack of the silicon-germanium oxynitride layer, a second portion of thefirst silicon oxynitride material, and a second portion of the secondsilicon oxynitride material can be patterned into a second gatedielectric, i.e., the gate dielectric layer 50 on the second body region32B. In this case, a thickness differential between the second gatedielectric and the first gate dielectric can be not greater than 0.3 nm.The thickness differential among various silicon-containing oxynitridematerial portions 52 can be under 0.3 nm for an entire range of SiGealloy composition including an atomic concentration of germanium from0.1% to 99.9%. Without wishing to be bound by a particular theory, it isbelieved that the formation of a thin initial (first) layer ofsilicon-containing oxynitride material in the silicon-containingoxynitride material portions 52′ completely passivates anysilicon-germanium alloy material underneath, and shuts down the role ofgermanium as a catalyst for enhancing the rate of the oxidation reactionduring the subsequent oxynitride growth.

The thickness of the first silicon oxynitride material layer 54 is thesame across different field effect transistors and irrespective ofunderlying dielectric material, i.e., irrespective of whether a siliconoxynitride is present underneath or a silicon-germanium oxynitride ispresent underneath. Further, the thickness of the second siliconoxynitride material layer 56 is the same across different field effecttransistors and irrespective of underlying dielectric materials. Thus,if the first top semiconductor portion 30A includes single crystallinesilicon, and if the second top semiconductor portion 30B includes asingle crystalline silicon-germanium alloy in which the atomicconcentration of germanium in a range from 20% and 30%, the thicknessdifferential between the gate dielectric layer 50 over the second topsemiconductor portion 30B and the gate dielectric layer 50 over thefirst top semiconductor portion 30A can be in a range from 0 nm to 0.3nm when the thickness of the silicon oxynitride in thesilicon-containing oxynitride material portion 52′ is in a range from0.7 nm to 1.2 nm.

In the first field effect transistor, a first gate dielectric (i.e., thegate dielectric layer 50 on the first body region 32A can be located ona silicon portion in a semiconductor substrate, and can include a firstvertical stack, from bottom to top, of a silicon oxynitride layer (i.e.,the silicon-containing oxynitride material portion 52 contacting thefirst body region 32A) including nitrogen at an atomic concentration ina range from 2% to 10% and a first silicon oxynitride material portion(i.e., the stack of the first silicon oxynitride material layer 54 andthe second silicon oxynitride material layer 56 over the first bodyregion 32A) having a nitrogen-to-oxygen atomic ratio greater than 1/3. Asecond gate dielectric (i.e., the gate dielectric layer 50 on the secondbody region 32B) can be located on a silicon-germanium alloy portion inthe semiconductor substrate, and can include a second vertical stack,from bottom to top, of a silicon-germanium oxynitride layer (i.e., thesilicon-containing oxynitride material portion 52 contacting the secondbody region 32B) including nitrogen at an atomic concentration in arange from 2% to 10% and a second silicon oxynitride material portion(i.e., the stack of the first silicon oxynitride material layer 54 andthe second silicon oxynitride material layer 56 over the second bodyregion 32B) having a same nitrogen-to-oxygen atomic ratio as the firstsilicon oxynitride material portion. If the silicon-germanium alloyportion includes germanium at an atomic concentration in a range from0.1% to 99.9%, a difference between the thickness of the second verticalstack and the first vertical stack can be not greater than 0.3 nm.

In one embodiment, a method of forming a semiconductor structureincludes forming a first gate dielectric (i.e., the gate dielectriclayer 50 on the first body region 32A) and a second gate dielectric(i.e., the gate dielectric layer 50 on the second body region 32B) overa first channel region in the first body region 32A and over a secondchannel region in the second body region 32B, respectively, in thesemiconductor substrate, which is the substrate 8. The first channelregion and the second channel region have different compositions, andthe first and second gate dielectrics have a substantially samethickness over the first channel region and the second channel region.As used herein, the definition of “having a substantially samethickness” means having thicknesses of which the difference does notexceed 0.3 nm. In one embodiment, the first and second gate dielectricscan have a substantially same thickness over the first channel regionand the second channel region, and the thicknesses of the first andsecond gate dielectrics may be not equal to each other.

A semiconductor structure according to an embodiment of the presentdisclosure includes a first channel region (i.e., an upper portion ofthe first body region 32A) location in a semiconductor substrate (i.e.,the substrate 8), a second channel region (i.e., an upper portion of thesecond body region 32B) located in the semiconductor substrate, a firstgate dielectric (i.e., the gate dielectric layer 50 on the first bodyregion 32A) located over the first channel region, and a second gatedielectric (i.e., the gate dielectric layer 50 on the second body region32B) located over the second channel region. The first and secondchannel regions have different compositions. The first and second gatedielectrics have substantially same thickness. In one embodiment, thefirst channel region includes silicon, and the second channel regionincludes a silicon-germanium alloy including germanium at an atomicconcentration greater than about 20%, and a difference betweenthicknesses of the second gate dielectric and the first gate dielectricis less than, or equal to, about 0.3 nm.

Referring to FIG. 8, a graph illustrating the relationship between aleakage current equivalent dielectric thickness and an inversion basedgate dielectric thickness of sample gate dielectrics formed by methodsof the present disclosure. The sample gate dielectric has a range ofvalues from 0.3 to 0.65 for the nitrogen-to-oxygen atomic ratio for theoverall composition, i.e., for the ratio of y to x in the formulaSiO_(x)N_(y) that represents the average composition within each of thesample gate dielectrics.

The leakage current equivalent dielectric thickness represents thethickness of a thermal silicon oxide that provides an equivalent leakagecurrent when used as a gate dielectric. The inversion based gatedielectric thickness of sample gate dielectrics represents the thicknessof a thermal silicon oxide that provides an equivalent inversion chargewhen used as a gate dielectric. The difference between the inversionbased gate dielectric thickness of sample gate dielectrics and thecorresponding leakage current equivalent dielectric thickness is about0.5 nm when the nitrogen-to-oxygen atomic ratio is 0.3, but decreases tovalues less than 0.4 nm when the nitrogen-to-oxygen atomic ratio is 0.35or above. Thus, by selecting a value for the nitrogen-to-oxygen atomicratio to be greater than 1/3, the difference between the inversion basedgate dielectric thickness of sample gate dielectrics and thecorresponding leakage current equivalent dielectric thickness can besignificantly decreased relative to a gate dielectric having anitrogen-to-oxygen atomic ratio of 0.3 or less. Thus, the gatedielectrics according to the present disclosure can provide a comparablelevel of inversion charge as prior art gate dielectrics, while providinga lesser electrical leakage than prior art gate dielectrics.Alternately, the gate dielectrics according to the present disclosurecan provide a comparable level of leakage current as prior art gatedielectrics, while providing a greater amount of inversion charges thanprior art gate dielectrics.

Referring to FIG. 9, a variation of the exemplary semiconductorstructure can be derived by forming at least one additional siliconoxynitride material on the gate dielectric layer 50L of FIG. 6 tothicken the gate dielectric layer 50L further. In this case, at leastonce a series of processing steps can be performed after the conversionof the second silicon oxide material to form the second siliconoxynitride material layer 56. Each series of processing steps caninclude deposition of an additional silicon oxide material on anunderlying surface of a silicon oxynitride material by an additionalatomic layer deposition process, and subsequent conversion of theadditional silicon oxide material into an additional silicon oxynitridematerial by an additional plasma nitridation process. The same ALDprocesses and the same plasma nitridation processes can be employed asin the processing steps of FIGS. 3-6. The gate dielectric layer 50Lwithin an increased thickness can have the same average composition asthe gate dielectric layer 50L shown in FIG. 6. The processing stepsillustrated in FIG. 7 can be subsequently performed to form field effecttransistors.

While the disclosure has been described in terms of specificembodiments, it is evident in view of the foregoing description thatnumerous alternatives, modifications and variations will be apparent tothose skilled in the art. Each of the various embodiments of the presentdisclosure can be implemented alone, or in combination with any otherembodiments of the present disclosure unless expressly disclosedotherwise or otherwise impossible as would be known to one of ordinaryskill in the art. Accordingly, the disclosure is intended to encompassall such alternatives, modifications and variations which fall withinthe scope and spirit of the disclosure and the following claims.

What is claimed is:
 1. A method of forming a semiconductor structurecomprising: providing a semiconductor substrate including a first topsemiconductor portion and a second top semiconductor portion;simultaneously converting an upper portion of said first topsemiconductor portion into a first semiconductor oxynitride portion andan upper portion of said second top semiconductor portion into a secondsemiconductor oxynitride portion by performing an oxynitration anneal,wherein said first semiconductor oxynitride portion and said secondsemiconductor oxynitride portion contain from 2 atomic % to 10 atomic %nitrogen; depositing a first silicon oxide material over said first andsecond semiconductor oxynitride portions by a first atomic layerdeposition process; converting said first silicon oxide material into afirst silicon oxynitride material employing at least a first plasmanitridation process; depositing a second silicon oxide material on saidfirst silicon oxynitride material by a second atomic layer depositionprocess; and converting said second silicon oxide material into a secondsilicon oxynitride material by a second plasma nitridation process,wherein a ratio of nitrogen atoms to oxygen atoms in a stack of saidfirst and second silicon oxynitride materials is greater than 1/3. 2.The method of claim 1, wherein said first atomic layer depositionprocess employs at least 10 deposition cycles, wherein each depositioncycle comprises: a silicon-containing precursor exposure phase in whicha process chamber containing said semiconductor substrate includes asilicon-containing precursor gas; and an oxygen exposure phase in whichsaid process chamber includes an oxygen-containing gas.
 3. The method ofclaim 2, wherein said first atomic layer deposition process employs anumber of deposition cycles, said number selected from a range from 12to
 20. 4. The method of claim 2, wherein said second atomic layerdeposition process employs at least 4 deposition cycles.
 5. The methodof claim 2, wherein said process chamber is void of anyoxygen-containing gas during said silicon-containing precursor phase,and is void of any silicon-containing gas during said oxygen exposurephase.
 6. The method of claim 2, wherein a molecule of saidsilicon-containing precursor gas comprises at least one silicon atom andat least one hydrogen atom.
 7. The method of claim 2, wherein saidoxygen-containing gas is selected from O₂, O₃, and a combination of O₂and O₃.
 8. The method of claim 1, wherein said first and second plasmanitridation processes employ a plasma of a nitrogen-containing gas. 9.The method of claim 8, wherein said nitrogen-containing gas is selectedfrom N₂, NH₃, NO, N₂O, NO₂, and combinations thereof.
 10. The method ofclaim 1, wherein said ratio of nitrogen atoms to oxygen atoms is in arange from 1/3 to 0.70.
 11. The method of claim 10, wherein said stackof said first and second silicon oxynitride materials has an overallcomposition of SiO_(x)N_(y), wherein x/2+3y/4 is in a range from 0.75 to1.25.
 12. The method of claim 1, further comprising annealing saidsemiconductor substrate with said first silicon oxynitride material inan environment including a nitrogen-containing gas or anoxygen-containing gas at an elevated temperature in a range from 700degrees Celsius to 1,100 degrees Celsius prior to said depositing ofsaid second silicon oxide material.
 13. The method of claim 1, whereinsaid converting is performed in an environment including anoxygen-containing gas or a nitrogen-containing gas at an elevatedtemperature in a range from 700 degrees Celsius to 1,100 degrees. 14.The method of claim 13, wherein said first and second semiconductorportions are each a silicon-containing material portion selected from asilicon portion and a silicon-germanium alloy portion.
 15. The method ofclaim 1, further comprising performing at least once a series ofprocessing steps after said converting of said second silicon oxidematerial, said series of processing steps comprising: depositing anadditional silicon oxide material on an underlying surface of a siliconoxynitride material by an additional atomic layer deposition process;and converting said additional silicon oxide material into an additionalsilicon oxynitride material by an additional plasma nitridation process.